This invention relates to induction motors. More particularly, the invention relates to a method and apparatus for indirectly measuring induction motor slip and using the measurement to establish speed control.
As an AC induction motor rotates, the magnetic fields of the rotor and the stator interact. The stator windings are typically connected to a supply in three-phase form or single phase form. By applying a voltage across the windings, a radial, rotating magnetic field is formed. The rotor has solid aluminum bars cast in a xe2x80x9csquirrel-cagexe2x80x9d configuration. The rotating magnetic fields produced by the stator produce a current in the aluminum bars of the rotor. This produces a magnetic field in the aluminum bars which interacts with the rotating magnetic field of the stator to generate torque on the rotor. The rotor reacts to the magnetic field, but does not travel at the same speed. The rotor actually lags behind the speed of the rotating magnetic field. This lag is called slip, and is essentially a comparison of the speed of the rotor and the speed of the magnetic field. The slip typically increases proportionately with increases in load.
In some applications where a variable speed drive (i.e., controller) is being used with an induction motor, it is desirable to establish a constant operating speed for the motor. However, the speed of the motor has a tendency to change depending upon the load applied to the motor. Therefore, to attain a constant speed with a changing load, the drive requires information about the actual rotor speed. Stated differently, it is necessary to provide electronic feedback of the rotor speed to the drive. In the prior art, this feedback to the controller is typically measured with a tachometer which can be both expensive and unreliable. Other methods utilize a model of the motor, in combination with phase voltage and phase current and complex mathematical algorithms to indirectly measure rotor speed. A much more complex and expensive circuit or microprocessor is required to perform these calculations.
Accordingly, the invention provides a method and apparatus for indirectly measuring induction motor slip and using the measurement to provide actual speed information to the drive to establish constant speed control of the induction motor. In the apparatus of the invention, measured analog DC bus voltage and analog DC bus current are input to a power factor circuit which uses the DC bus voltage and current along with AC motor voltage and current to calculate a power factor. The power factor circuit is connected to a slip calculator circuit which uses desired motor speed and power factor to calculate slip. That is, since the desired speed of the motor is known, the measured power factor and speed can be used to calculate the slip, thereby providing an indirect measurement of induction motor slip. The slip calculator circuit is connected to a speed conversion circuit which converts the indirect measurement of induction motor slip to an actual motor speed value. This value is xe2x80x9cfed-backxe2x80x9d to the regulator to provide feedback control to the drive which constantly updates the commanded drive speed until the commanded speed and the actual speed (also referred to as xe2x80x9cestimated speedxe2x80x9d or xe2x80x9cmeasured speedxe2x80x9d) are equal.
In the method of the invention, three phase AC power is supplied to energize the motor. A DC bus voltage and a DC bus current are measured. The measured DC bus voltage and current along with AC motor voltage and current are used to calculate the power factor for the commanded motor speed. The commanded motor speed and power factor are then used to calculate the motor slip at the given speed and power factor. Stated differently, the induction motor slip is indirectly measured based on the DC bus voltage and current. This technique is most effective for variable speed drives at higher speeds for two reasons. The first is that there are two slips for each power factor value, one at relatively low speeds and a second at relatively high speeds. The second is that, at different drive frequencies, the stator resistance becomes a larger percentage of the loss impedance of the motor thus changing the power factor to slip relationship. At higher speeds, the influence of the stator resistance on this relationship is nominal.
The slip is then used to calculate an actual motor speed value. This value is xe2x80x9cfed-backxe2x80x9d to the regulator to provide feedback control to the drive which constantly updates the commanded drive speed until the commanded speed and the actual speed (also referred to as xe2x80x9cestimated speedxe2x80x9d or xe2x80x9cmeasured speedxe2x80x9d) are equal.
The principal advantage of the invention is to provide constant speed control of an induction motor by indirectly measuring the motor slip.
Another advantage of the invention is the use of measured bus voltage and bus current in addition to phase voltage and phase current to simplify the rotor speed calculation. The simpler calculation allows the use of a simpler and less expensive microprocessor circuit.